1. Field of the Invention
The present invention relates generally to systems and methods for light pulse positioning or steering, and particularly to systems and methods for laser beam scanning using acousto-optical deflectors (AOD). The invention has applications in various fields including laser imaging such as multi-photon laser scanning microscopy, laser micromanipulation such as laser micromachining and laser ablation, and optical storage such as CD writing and/or reading.
2. Description of the Prior Art
Laser has been used for microimaging, micromanipulation and optical data systems in a variety of ways. Laser microimaging found in modern optical instruments is playing an increasingly important role in the study of physical, chemical, biological and medical processes and systems. On another front, laser micromanipulation, including but not limited to laser micromachining and laser ablation, has become a critical part of semiconductor fabrication, Micro-Electro-Mechanical Systems (MEMS) and nanotechnology.
Whether it is for imaging, manipulation or optical data system, a general and key aspect of this laser technology is to steer a laser beam to a desired target position in a workspace. Because generally the workspace that needs to be imaged or manipulated is much greater than the size of the laser spot, a scanning mechanism is used to scan the workspace by systematically moving the laser beam and the associated laser spot across the workspace. Conventionally, the steering or positioning of a laser beam is performed by a moving mirror, which is typically done through a galvanometer, which is a device for detecting or measuring a small electric current by movements of a magnetic needle or of a coil in a magnetic field. Galvanometer-driven mirror, although still widely used in laser scanning devices, has its limitations, among which are slow scanning speed and inability or difficulty to position the laser beam non-sequentially or randomly. The scanning speed enabled by a galvanometer-driven mirror is typically only about several frames per second, too slow to image the fast response of biological samples. For example, an image rate of 1 KHz is required to image the membrane potential by using voltage sensitive dyes.
To overcome the shortcomings of the conventional galvanometer-driven mirror scanner, other scanning methods have been proposed. Among them, scanning using acousto-optic deflectors (AOD) may be most promising. The advantage of using AOD as a scanner or positioner is that it has no mechanical moving part, and thus has the potential of random accessibility (steering or positioning the laser beam rapidly to any point in the view of field (FOV)) with high precision and repeatability.
One significant technical issue of using AOD as a scanner or positioner is that AOD is highly dispersive. Although laser is generally considered monochromatic, pulsed laser has multi-chromatic components because of its pulsed (truncated) nature as opposed to a single wavelength continuous wave. The dispersion caused by AOD limits its applications. Spatial dispersion of laser pulses directly affects the resolution of laser imaging or laser manipulation. Temporal dispersion of laser pulses causes additional problems. For example, in multi-photon microscopy, dispersion can significantly degrade the efficiency and quality of multi-photon excitation which is the basis for this type of microscopy.
Due to the potentials and promises of using AOD for pulsed laser scanning or positioning, much effort has been made to improve the technology. U.S. Pat. No. 6,804,000 B2 to Roorda et al, for example, discloses a method for laser beam steering using AOD and dispersion compensatory optics. A dispersive element, such as a prism, is placed along the path of the monochromatic light pulses to disperse the multi-chromatic light pulses in the direction opposite to the spectral dispersion caused by the AOD. The basic design of the above-referenced patent is for one-dimensional scanning using one AOD. For two-dimensional scanning using two AODs, it is suggested that the basic design for one-dimensional scanning be simply duplicated. That is, two separate compensatory prisms are used, one for each AOD, to scan in its corresponding dimension. Furthermore, the design of the above-referenced patent is to address spatial dispersion only. For temporal dispersion, it is suggested that the conventional prism-pair (two prisms) method, which is first demonstrated in 1984, be used.
U.S. Pat. No. 6,555,781 B2 to Ngoi et al discloses a method and apparatus for precision laser scanning in which an acousto-optical modulator or diffraction grating is used to compensate the spatial dispersion of a two-dimensional AOD. The design in this patent does not address temporal dispersion.
Despite the efforts, reliable and economic two-dimensional AOD laser scanner with true random positioning capability is yet to be realized. In the existing microscopes, AOD is generally used to accomplish scanning only along a fast scanning axis, while scanning along the other axis is still accomplished with galvanometer-driven mirrors.
Given the importance of laser imaging and laser manipulation, it is desirable to develop a new laser scanning or positioning system that is faster, easier and cheaper to construct, and has better resolution and real two-dimensional random accessibility.